The finishing system of choice presently being used on the exterior of automobiles and trucks comprises a clear coating applied over pigmented base coating that is applied over a primer coating. The clear coating provides protection, in particular, protection from weathering, to the pigmented base coating and improves the appearance of the overall finish, in particular, provides improved gloss and distinctness of image. The primer coating provides adhesion to the substrate and, in particular, provides resistance to stone chipping. When used in refinishing of automobile and truck bodies, the clear coating is required to have an acceptable “pot life” and reasonably short cure time period to allow for further processing or handling of the vehicle without damaging the finish. The term “pot life” means the period of time after a coating is mixed with a catalyst or a crosslinking agent in which the composition remains at a sprayable viscosity.
The following U.S. patents: U.S. Pat. No. 5,516,873, U.S. Pat. No. 5,126,170, U.S. Pat. No. 5,243,012, U.S. Pat. No. 5,236,741, U.S. Pat. No. 5,412,056, U.S. Pat. No. 5,580,945, and U.S. Pat. No. 6,005,062, show a variety of coating composition that contain polyaspartic acid derivatives but these compositions do not have a property balance of acceptable pot life and rapid curing time to form a sufficiently hard finish to allow additional handling and processing of a coated vehicle or work piece after the coating composition has been applied.
To improve the rate curing, EP 0939091 uses novel amine compounds, for example, the reaction product of 4,4′-methylene-biscyclohexanamine with two moles of diethyl maleate. However, coating composition formulated with these reactive amines do not have the desired balance of acceptable pot-life and the desired cure rate after application to an object while maintaining or improving on the desired properties of the resulting finish. In an effort to improve pot life, solvents and catalysts have been used but solvents have a deleterious effect on VOC (volatile organic content) emissions, which is undesirable and catalyst can result in deterioration of film properties, such as durability. It is, therefore, desired to find a class of amine functional compounds for the reaction with isocyanates, which form coating compositions that overcome these problems and form acceptable finishes for automotive and truck substrates.
EP 0743333 describes the use of simple hydroxy aspartates in the preparation of hydroxy-functional polyhydantion prepolymers and their use as co-reactants for blocked polyisocyanates or aminoplast resins. The use of hydroxy-aspartates as the nucleophilic component in coating systems with polyisocyanates is not described. In U.S. Pat. No. 5,561,214, the use of hyperbranched polyaspartate ester polymers is described based on the selfcondensation of hydroxy-aspartates and these polymers are used as binder resins in coating systems.
The novel composition of this invention utilizes novel reactive amine compound having less reactive hydroxy functional groups as a nucleophilic component with a polyisocyanate crosslinking agent that form coating compositions having an optimum balance of pot life and curing time and form finishes, in particular, clear and primer finishes useful for automobiles and trucks. The clear coatings have excellent properties, such as, hardness, gloss, durability, weatherability, and in particular resistance to UV (ultraviolet light) degradation, particularly when reinforced with ultraviolet light absorbers and screeners and hindered amine light stabilizers. The primer coatings exhibit excellent adhesion to metal substrates, in particular, aluminum and steel substrates, and provide for excellent stone chip resistance.
These compounds may be used as part of the binder component or as the sole nucleophilic component in a two component coating mixture. When used as the sole nucleophile, especially environmentally friendly coating compositions may be formulated with low or even zero VOC (volatile organic content). The advancement achieved with the novel coating compositions of this invention over the prior art is based on effectively balancing the pot life of the coating mixture with the cure characteristics using the unique structural characteristics of new aspartic-hydroxyl compositions in combination with polyisocyanates. It is surprising that the use of a nucleophilic component based on compounds of this invention which contain both aspartate and hydroxyl functional groups in a curing reaction with polyisocyanates leads to a high productivity coating system with good general coating properties while meeting desired environmental goals of eliminating or reducing VOC of a coating composition while maintaining a good pot life.